Those skilled in the art are aware of the manufacture of composite panels including a central core structure which is provided with face sheets on the opposite surfaces thereof. Such panels and core structures therefor are shown in Campbell previously issued U.S. Pat. Nos. 2,930,882; 3,015,715; 3,077,532; 3,598,953; 3,689,730; and Johnson Pat. No. 2,983,038.
Campbell previously issued patents teach, among other concepts, the utilization of a core structure which consists of a plurality of internested core strips incorporating continuous flanges provided with male and female nodes adapted to internest with each other to provide a core of the desired area.
It is also known to provide core structures which include a plurality of core strips incorporating a series of tabs which are defined by a plurality of notches in the opposite edges of the core strip. Typical of the use of such notched core strip is the Johnson patent mentioned hereinabove.
Also known to those skilled in the art is the provision of panels of the Johnson-type wherein the apices of the corrugations of the core strips are notched to facilitate the deformation of the core strips into the corrugated configuration and the internesting of the core strips with one another. When such notched core strips are utilized, no attachment surfaces are provided at notched portions of the core strip for attachment to the inner surfaces of the juxtaposed face sheets of the panel. Consequently, the resistance of the resultant panel to flatwise tension is reduced by 15 to 20%.
The conventional process of forming core strip blanks initially entails the slitting of ribbons or strips of metal from relatively wide sheets of material. The slitting process results in the formation of minute cracks in the opposite edges of the resultant strip or ribbon and also causes the work hardening thereof which entails the necessity for an annealing process subsequent to the slitting of the blank.
However, even though the blank is annealed, micro-cracks remain in the opposite edges of the stainless steel materials utilized in the fabrication of the blanks, such as, Inco 718, 316, 347, 625, Rene 41, etc.
Consequently, when the blanks are subjected to the extreme deformation entailed by the internesting of the male and female nodes characteristic of the core structures of the aforementioned Campbell patents, the micro-cracks are greatly enlarged thus resulting in substantial reduction in the load bearing characteristics of the resultant core structure and possible cracking of the same when deformed in forming dies and/or subsequent handling.
Reference is made to our co-pending applications for U.S. Pat. Ser. No. 948,012, entitled Metallic Core Panel and Method of Making Same, filed Oct. 2, 1978, and Ser. No. 002,761, entitled Apparatus for Fabricating and Welding Core Reinforced Panel, filed Jan. 12, 1979.